When fixing semiconductor chips to a substrate by soft soldering, the required portion of solder has to be placed on the substrate before positioning the chip. A method often used for this purpose is the touching with one end of the soldering wire, of the respective spot of the substrate--typically a lead frame made of copper alloy--with the substrate heated to a temperature above the melting point of the solder, thus melting a piece of the wire. Another known process is the placing of discrete, solid solder portions on the heated substrate. In both cases, the liquid solder portions develop are more or less dome-shaped, with shape and position of the drop of molten solder depending very much on the local wetting characteristics of the substrate's surface, among other factors.
For faultless soldering of the chip placed on the substrate later, however, a flat solder layer distributed under the entire chip surface and of a thickness as even as possible is required. The dome shape mentioned and possible lateral deviations in the positioning of the individual drops make it difficult, however, to meet all these requirements reliably and with sufficient accuracy. It also has to be taken into account that the assembly of chips is performed at a high rate, which means that the chips are in general placed at high speed on the substrate, i.e. on the liquid solder.